It sounds like you probably have Hashimoto's or Graves disease, both are autoimmune disease where you develop antibodies that attack your thyroid, and both can cause elevated levels of thyroid peroxidase. Did they check anti-thyroglobulin (which is high in Hashimoto's) or thyrotropin receptor antibody (which is high in Graves)?
Graves disease causes hyperthyroidism, Hashimoto's most often causes hypothyroidism. Regardless of the thyroid nodules, if you have hypothyroidism or hyperthyroidism, you may need treatment.
On to the thyroid nodules - I have Hashimoto's, and I had a "multinodular goiter", which is a thyroid full of nodules. I don't know if you just have a few nodules on each lobe or if your thyroid is like mine was, full of nodules. This can be caused by Hashimoto's or Graves, and at least with Hashimoto's, the cause of the multinodular goiter is repeated rounds of antibodies and immune cells destroying thyroid tissue, the thyroid then makes less thyroid hormone, which causes the pituitary gland to secrete more TSH (thyroid stimulating hormone), which causes the thyroid to grow. I believe multinodular goiter usually happens when Hashimoto's has been attacking the thyroid for a long time. (I discovered my multinodular goiter and Hashimoto's around the same time in 2018, but I had a ton of hypothyroidism symptoms for years before a diagnosis.)
Thyroid nodules are extremely common and most are not cancerous (something like 90-95% are not cancerous). However, depending on the size and texture of your nodules on the ultrasound, they may want to follow up with a fine-needle biopsy to determine whether it is benign/supsicious/malignant, and depending on the outcome, either continue to monitor the nodules by ultrasound, or do something else. Current guidelines usually wait to do a biopsy until a nodule is 1.5-2 cm, since above 2 cm increases the likelihood of cancer, if the texture is different from the rest of the thyroid (e.g. hypoechoic), that can also increase the cancer risk. Having nodules and having a multinodular goiter does not mean it is cancer, but it is something that needs to be monitored (for example, an ultrasound each year to measure growth, or a fine needle biopsy to make sure that it is benign).
What do you need to do? Two things:
1.) make sure your thyroid hormone levels are checked - not just TSH, TSH can indicate a problem but it is often "in range" even when a problem is occurring - you should probably have them check free T4 and free T3, the inactive and active form of thyroid hormone. If your free T4 and free T3 are in the bottom half of their range or lower, it might indicate hypothyroidism. If they are high, it might indicate hyperthyroidism. You can also look at a list of hypothyroidism and hyperthyroidism symptoms (found many places on the internet) and see if you might be experiencing some of them. If you are experiencing hypo- or hyperthyroidism, you may need to get treated for that, independent of what is going on with your thyroid nodules (Hypothyroidism you can take supplemental thyroid hormone - hyperthyroidism is a little trickier to treat.) When I discovered my multinodular goiter, my TSH was in the normal range, my free T4 and free T3 were towards the middle /lower parts of the normal range. My doctors refused to let me try supplemental thyroid hormone (until a couple months later when the thyroid had to come out anyway).
2.) Follow up on your thyroid ultrasound - find out what the ultrasound report recommends for a follow-up and ask your doctor if you need a follow-up ultrasound. If you have a multinodular goiter, the nodules need to be monitored every so often (1-2 years) to see if they are growing. If I had no suspicious nodules (nodules that weren't a certain size or certain texture), I would have done follow-up ultrasounds every year. One of my nodules was about 1.3 cm with hypoechoic texture, which made it more likely to be cancer, so I went for a fine needle biopsy and ended up needing to remove my thyroid. That in no way means yours is cancerous, just that now that you know the nodules exist, you may need follow up, either with ultrasounds to see if the nodules increase in size, or a biopsy if there are suspicious nodules. It could be that your nodules are all small and a fine needle biopsy was not recommended at the time it was done 18 months ago. From what I can find, it looks like serial ultrasounds to follow size are recommended every 6-18 months after a fine needle biopsy comes back benign.
Good luck. If you have the thyroid ultrasound report and need help interpreting it, feel free to share it here. Usually, if the nodule size is below 1 cm, a fine-needle biopsy is not recommended, no matter the ultrasound texture.